A left axis deviation means that the overall direction of ventricular depolarisation is abormal and too far around towards the left shoulder. This direction can be between -30 degrees (up towards the left shoulder) to -90 degrees (vertically upwards) from horizontal. If the axis is leftward, Lead I will be positive, aVF negative and lead II negative.
Left Axis Deviation can be caused by lots of things, including left ventricular hypertrophy, left bundle branch block, inferior MI, ventricular pacing, and others. The diagnosis of exclusion is left anterior fascicular block.See also: Axis
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This ECG is from a man in his 50s who presented with syncope on a background of diabetes, opiate abuse and possible prior cardiac disease.
This ECG shows a supraventricular tachycardia with evidence of P waves, left axis deviation and slightly wider QRS (106 msec) likely due to LAFB, tall QRS concerning for LVH and fragmentation in V2-4 suggesting prior MI.